At the present time metal cans are often produced as "two piece cans" which consist of a cylindrical can body with an integral bottom wall and a can top. Millions of such cans are made each day. They are generally made of thin aluminum or steel sheet metal. For example, aluminum cans are used to pack gas pressurized liquids, such as beer and soda. The can must have a certain strength so that it can withstand internal gas pressure as well as the pressures from stacking, dispensing machines and handling. However, thickness of the sheet metal is an important part of the cost of such cans. If the metal may be made thinner, while retaining the required strength, then the cans may be produced at a decreased cost.
One way to obtain can strength, using sheet metal, is to form circular curves in the can bottom. Such curves, viewed from the bottom of the can, are one or more concentric circles in the can bottom and/or can side wall near the bottom. Seen in cross-section, such curves are rounded, generally in a hemispherical shape.
The machine which forms the shape of the can body is sometimes called a "necker". It operates by applying pressure to the can body after it has been formed into its general body shape, e.g., a cylinder or multi-angular shape with an integral bottom wall.
The necker is a type of tool and die in which the sheet metal is placed between the tool, having a protrusion, and the die, having a matching indentation. The tool and die are brought together, under pressure forcing the sheet metal to assume the shape of the protrusion-indentation.
Cans are produced at high speed. For example, the BELVAC (TM Belgium Tool & Die Co.) may form can bodies at the speed of up to 2500 cans per minute. The can bodies are squeezed ("necked") between opposite moving ram assemblies. A series of push rams act as tools and an opposite series of knockout rams act as dies.
The ram assembly can be installed in a rotating drum-shaped component, the turret. The tooling and forming side turrets are rotated at the same speed, so that the can, while engaged in the necking operation, rotates with the turret but is stationary relative to the ram assemblies. A ram assembly has a "bushing" (cylinder), which is a fixed housing, and a ram (piston) which slides within the bushing. The can bodies, as they are progressed through the machine, are rapidly squeezed between a first push ram and a first knockout ram, then a second push ram and second knockout ram, for as many as six or eight pairs of push rams and knockout rams to complete the "necking" operation. Each ram, at its end, has a hard metal disk (ram die) which forms the can's curves.
Each of the rams operates back and forth at high speed and with a travel distance of 1-3 inches. The rams are mechanically operated, as an air pressure or electromechanical solenoid system would be too fragile for the required high pressures and speeds and extreme shock loading. The rams must move with an exact timing, without binding, i.e., ram alignment is critical. If a ram becomes jammed or becomes worn, it may fail and hold up the entire can production system. For that reason the rams are carefully lubricated, inspected and replaced. In older machines the rams were manually lubricated, which was a messy, labor-intensive and time-consuming process. More modern machines use automatic lubrication which involves running grease tubes to each ram. Since the ram assemblies are attached to a turret which rotates, a special coupling for the grease supply line is necessary to distribute the grease to each ram. These couplings often wear out, allowing grease to escape. However, even in assemblies with automatic lubrication, the grease may end up on the can bodies, preventing paint adhesion. No one wants a greasy can of beer. Even with proper lubrication, the rams may have to be inspected and replaced every few weeks, for example, every two weeks in higher speed lines, which is a large expense in terms of parts, machine down-time, the time of skilled labor, and scrapped cans.
The ram pistons are generally round (in cross-section) and have a raised key (elongated protrusion) at its top. The key slides in a key-way (slot) formed in outer body, a bushing-like part. That key and key-way slot system prevents the ram from turning about its axis. However, the key-ram is under considerable sliding pressure and often is the first part of the ram assembly to fail. The ram assembly is made of strong and heavy metals, for example, steel and brass. A ram assembly may weigh 20 to 40 pounds, for example, a typical BELVAC ram assembly weighs about 20 pounds. The piston of the ram assembly, made of steel, itself weighs over 10 pounds. That heavy ram (piston) must be pulled and pushed back and forth rapidly and exactly (in time), which requires a strong machine to operate the cam. The cam is an elongated raised track (rail) which is curved. Each ram (piston) carries, at its rear end, two wheels which fit on opposite sides of the cam track. As the cam track turns it pulls and pushes the wheels causing the ram (piston) to move forward and backwards.
In U.S. Pat. No. 5,467,628, assigned to Belvac Production Machinery, incorporated by reference, a sliding bushing 20 surrounds a ram 22 in a can bottom reprofiler machine. The tail end of the ram carries cam follower wheels 56 which ride against a cam track 57. This general type of double-ram can forming machine is also shown in U.S. Pat. Nos. 4,732,027 and 4,272,977, also incorporated by reference.